It has for a number of years been common practice in air craft wing design to arrange different types of winglets or other means at the wing tip for reducing or preventing the spanwise flow of air from the pressure (lower) side of the wing profile to the suction (upper) side of the profile around the tip, which results in the creation of the tip vortex and a decreased lift coefficient at the tip section of the wing, mainly due to the reduced suction at the suction side. Wind turbine rotors having blades equipped with winglets are also known in the art, mainly for the purpose of reducing noise emission from the wind turbine due to the presence of the tip vortices but also to improve the overall performance of the wind turbine.
WO 2004/061298 A2 discloses such blade for a wind turbine, where a particular design of the winglet itself is disclosed.
EP 1 500 814 A1 shows a wind turbine blade with an end projection having an aerodynamic cross-sectional profile, which lies in a plane extending at an angle to the rotor blade plane. The end projection is asymmetric to the central longitudinal axis of the rotor blade, with a progressive or stepped reduction in the blade thickness at the transition between the end projection and the remainder of the rotor blade.
WO 2005/078277 A2 relates to a rotor blade for a wind turbine with a drag ratio, in particular in the central or main board region of said rotor, whose value exceeds 80% and preferably 90% of the maximum value of said ratio in the range of +/−20 of the optimum pitch of said rotor.
One of the consequences of producing lift on a finite wing is the generation of spanwise flow around the tip which influences the flow pattern in the whole tip region. In particular, the pressure gradients caused by the lower pressures on the upper surface relative to the higher pressures on the lower surface lead to inward spanwise flow (toward the hub) on the upper surface and outward spanwise flow (toward the tip) on the lower surface. At the trailing edge, the merging of these two flows having different directions generates the vorticity that is shed from a finite wing and is the origin of induced drag as well as aerodynamic noise.
An endplate at the tip of a finite wing can reduce the spanwise flow and thereby reduce the induced drag. Unfortunately, to be effective, the endplate must be so large that the increase in wetted area drag far outweighs any drag reduction. A winglet, rather than being a simple fence which limits the spanwise flow, carries an aerodynamic load that produces a flow field i.e. an inward side force that allows its own induced velocity field to partially cancel that of the main wing, thereby reducing the amount of spanwise flow. In essence, the winglet diffuses or spreads out the influence of the tip vortex such that the downwash and, in turn the induced drag, are reduced. In this way, the winglet acts like an endplate in reducing the spanwise flow but, by carrying the proper aerodynamic loading, it accomplishes this with much less wetted area.
The displacement of the wing tip out and away from the main wing planform reduces the effect of the shed vorticity on the wing by displacing the concentrated vorticity away from the wing. In this manner, the winglet emulates the effect of a planar span extension and an increase in the length of the load perimeter.
The diffusion process is also realized as an expansion of the wake in the far field due to induced velocities from the non-planar components of the winglet. The out of plane bound vortex on an upward winglet induces horizontal velocities on the free wake that cause a spanwise spreading of the wake field. This also emulates the effect of a span increase.
Another benefit of winglets, which is not achieved by a simple span extension, is the effect on the spanwise lift distribution, particularly in the region of the wing tip. The influence of the winglet effectively loads the planform in the tip region, increasing the local lift coefficients and filling out the spanwise lift distribution. Planform efficiencies greater than those of an elliptical wing are possible. This occurs because, as evidenced by the extension of the roughly constant lift coefficients to beyond the actual tip location, the tip loaded spanwise lift distribution is, in fact, behaving like that of a nearly elliptically loaded planform of a greater span. When referenced to the actual span, the resulting efficiency is greater than that of an elliptical loading.
Summering up the overall benefits of winglet vs. tip extension is:                1. Installation of winglets is found to cause a larger increase in the power coefficient and a smaller increase in the flap bending moment than radially extended rotor blades        2. The smaller turbine diameter for the same tip velocity results in smaller gear ratio.        3. On some sites the local regulations dictates maximum wind turbine height (tower+blade tip in highest position).        4. Decrease noise from tip vortices.        